Hydraulic machines, for example, hydraulic excavators, use engines to drive variable displacement pumps, which provide hydraulic power to cylinders. The engine often operates at a fixed high speed, regardless of the actual instantaneous power requirement of the machine. Thus, even when power demand is low, the engine runs at an inefficient high speed, resulting in higher than necessary fuel consumption and wear on the engine. However, when power demand is high, the engine is already running at a high speed and the necessary power can be delivered to the hydraulic system. Thus, the engine runs at an optimal speed for high power demand tasks, but runs at an inefficient speed for lower power demand tasks.
One exemplary hydraulic machine is a hydraulic excavator, which is useful for a number of tasks, which can be assessed as distinct steps. Excavators are often used to dig trenches. During a typical dig cycle, the excavator begins at the dig step by digging with its bucket into the soil. Next, during the lift and swing step, the excavator lifts the soil into the air and swings towards the dump location, for example a waiting dump truck. During the dump step, the machine dumps the soil at the dump location. Finally, during the return step the excavator swings back to the dig location, while lowering the bucket, and thus is ready for the next dig cycle. During the entirety of the dig cycle, the machine runs at maximum power. However, only the dig step and the lift and swing steps require high engine power. The dump and return steps require less power, but the machine typically runs at high power, thus unnecessarily consuming fuel.
Simple control schemes have been implemented to lower the engine speed of hydraulic machines and thus to conserve fuel during low power requirement operations. For example, engine speeds have been reduced to idle during sustained periods of waiting to conserve fuel. However, this control scheme does not vary the engine speed during active work cycles when less than full engine speed and hydraulic pump flow are required.
More sophisticated control schemes have been used to vary engine speed during an active work cycle. One exemplary method and apparatus for controlling engine speed during active work cycles is disclosed in U.S. Pat. No. 5,967,756 to Devier et al. The disclosed method varies the engine speed of a hydraulic machine based on the desired pump displacement and the desired engine speed. The engine speed is reduced to an efficient engine speed based on the desired pump displacement. While this method provides a more efficient machine, it does not optimize the efficiency of the machine based on the actual work being performed by the machine. Therefore, the disclosed method does not achieve the best optimizations.
The foregoing background discussion is intended solely to aid the reader. It is not intended to limit the invention, and thus should not be taken to indicate that any particular element of a prior system is unsuitable for use within the invention, nor is it intended to indicate that any element, including solving the motivating problem, is essential in implementing the innovations described herein. The implementations and application of the innovations described herein are defined by the appended claims.